Point to Multi-Point (PMP) Digital Radio Relay Systems (DRRS) are well known. A typical PMP DRRS system 10 is shown in FIG. 1. System 10 generally comprises a base station (BS) 12 and a plurality of remote stations (RS) 14-1, . . . , 14-N. Each of base station 12 and remote stations 14-1, . . . , 14-N is equipped with a transceiver and antenna for communication. The system of FIG. 1 is very general and the base station 12 may be either a ground-based central office or satellite transceiver. The remote stations 14-1, . . . , 14-N may be cellular telephones, remote fixed locations or mobile or fixed switchboards or central offices. Depending on the type of system, each of base station 12 and remote stations 14-1, . . . , 14-N may also transmit the information between a user and another station.
The radio communication links shown in FIG. 1 are two way, and communication between the base station 12 and the remote stations 14-1, . . . , 14-N is generally two way or duplex. Generally, this duplex communication comprises a down-link or forward link 16-1, . . . , 16-N (BS to RS) and an up-link or return link 18-1, . . . , 18-N (RS to BS). Generally one of two duplex methods is used, Frequency Division Duplexing (FDD) or Time Division Duplexing (TDD).
In FDD, duplexing separation is achieved by using different frequencies for the up-link and down-link. In many cases, FDD systems are required by regulatory bodies to operate within a frequency channel plan using a fixed frequency difference between the up-link and the down-link. In order to achieve wide range transmission, the transmission power of radio relay systems may be as high as +10 to +30 dBm and the sensitivity can be as low as −90 to −110 dBm. Therefore, well over 100 dB isolation is typically provided between transmitter and receiver. This is typically achieved by frequency separation and filtering. Further, in systems in which the same antenna is used for transmission and reception, a circulator is also used. Since the frequency separation between transmitter and receiver may be as low as a few percent, the filters are usually large and expensive devices.
In TDD systems, duplex separation is achieved by transmitting the up-link and down-link in different time slots. TDD does not require isolation between the transmitter and receiver (although a TR device or the like may be required to provide overloading of the receiver when the transmitter is operating). Furthermore, the receiver and transmitter may share certain hardware such as filters and RF sources.
Another aspect of PMP DRRS is the multiple access method used to allocate time/bandwidth so that a plurality of remote stations 14-1, . . . , 14-N can communicate with the same base station 12 without interfering with each other. Several methods are in general use. These include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). The access method is intended to avoid interference between the up-links. No such problem exists for the down-links; however, the common practice is to employ the same arrangement of “access” on the down-links as on the up-links.
In FDMA, the base station 12 communicates with each remote station 14-1, . . . , 14-N using at least one unique radio frequency. In general, the total bandwidth is separated into channels each having a given bandwidth. One or two channels are assigned to each of the remote stations 14-1, . . . , 14-N for use in duplex transmission between the remote stations 14-1, . . . , 14-N and the base station 12. In FDD, a separate frequency channel is assigned to a given up-link 18-1, . . . , 18-N and its associated down-link 16-1, . . . , 16-N. In TDD the same frequency bandwidth is assigned to the associated down-link 16-1, . . . , 16-N and up-link 18-1, . . . , 18-N for each remote station 14-1, . . . , 14-N. For the same total transmission capacity, the total bandwidth assigned to a channel is the same whether FDD or TDD is used.
In TDMA, time is divided into repeating time frames. Each time frame is separated into time slots one of which is assigned to each link. In FDD, the time slots at a given frequency are used for up-links 118-1, . . . , 118-N or down-links 116-1, . . . , 116-N, with similar time slots at a different frequency being used for the other links. In TDD, half of the time slots are used for up-links 118-1, . . . , 118-N and half are used for down-links 116-1, . . . , 116-N.
FDMA has certain advantages over TDMA. In particular, FDMA signals are transmitted continuously (for FDD) or half the time (for TDD). In TDMA, transmission takes place only during the allotted slot. Thus, for the same average transmission, a higher peak transmission rate is required for TDMA. For this reason, the sensitivity of TDMA systems are lower and higher peak transmission power is required for the same system gain. Due to the lower transmission rate, FDMA systems are typically less susceptible to signal distortion caused by selective fading than TDMA systems.
TDMA has certain advantages over FDMA. In particular, due to the naturally interrupted nature of the links, it is easier to allocate the time slots and to change the allocation. Even when FDD is implemented, it is sometimes possible to arrange the time slots (see FIG. 2) such that a given remote station 14-1, . . . , 14-N does not send and receive at the same time. In those cases, the system 10 has the benefits of a TDD system in that there is no need for a duplexer, nor for isolation between the transmitter and receiver of the remote stations 14-1, . . . , 14-N. Furthermore, there is the possibility of hardware sharing in the remote stations 14-1, . . . , 14-N as described above.
One of the requirements for every communication system is the synchronization of the transmitter and receiver. One or more of carrier frequency synchronization, carrier phase synchronization (for improved detection sensitivity) and symbol (rate and phase) synchronization are required for proper transmission and receiving of communications.
Generally, synchronization is performed in two steps. These steps are acquisition, performed at link set-up and when frequencies are changed and tracking, based on an analysis of the signals received during regular transmission. While acquisition way take a long time, due to uncertainty of the characteristics of the signal being acquired, time may be reduced by use of special training sequences. In general, acquisition is performed in three consecutive steps, namely, carrier frequency acquisition, timing clock acquisition (frequency and phase) and carrier phase acquisition (if required). In FDMA systems that operate in FDD, transmission is continuous so that acquisition is required only at link set-up. In FDMA systems that operate in TDD and in TDMA systems the transmission is not continuous such that the signal must be reacquired for each transmission (frame). The acquisition is usually performed on a training sequence or preamble, which, since it does not carry information, results in a reduced transmission capacity. This is another advantage of FDMA/FDD systems.
Multi-user communication systems generally operate with a fixed overall bandwidth allocation. In some systems, a Fixed Assignment (FA) of bandwidth is used. This may be a uniform assignment, in which each remote stations 14-1, . . . , 14-N receives the same communication bandwidth (or length of time slot) or non-uniform assignment (in which each RS receives a different bandwidth). Other systems utilize a Demand Assignment (DA) in which bandwidth is dynamically assigned for each remote station 14-1, . . . , 14-N in accordance with its current requirements for service.
Data rates of the individual links may be uniform or non-uniform, depending on the service provided to and by the remote stations 14-1, . . . , 14-N. For example, if each remote station 14-1, . . . , 14-N provides service to a single line, and all lines are of the same kind, the data rates would generally be the same for all the links. On the other hand, multi-line remote stations, with different types or numbers of active lines or different kinds of services would require different data rates (which implies different bandwidths), for the different remote stations 14-1, . . . , 14-N.
In DA, data rates or bandwidths can be assigned statically (fixed so long as the link is operating) or dynamically (changed during operation of the link without any break in service). For example, the data rate or bandwidth would be changed dynamically if an additional subscriber served by a remote station 14-1, . . . , 14-N became active or if a subscriber hung up.
Modulation rates of the individual links may be uniform or non-uniform, with non-uniform rates being used to adjust each link to operating conditions, such as distance and propagation conditions. For example, the modulation rate may be changed to add error-correcting schemes when the propagation conditions are poor. Modulation rates can be static or dynamic. Non-uniform and dynamic modulation rates are addressed, for example, in “Modem Quadrature Amplitude Modulation,” IEEE Press, NY, 1994, Chapter 13.
The data rate and the modulation rate of an individual link determine its symbol rate and the symbol rate determines the bandwidth occupied. Thus, the bandwidth assignment (frequency slots in FDMA or time slots in TDMA) can be uniform or non-uniform, static or dynamic.
In TDMA, with FA and DA, uniform and non-uniform static and dynamic assignment of time slots are easily implemented in an efficient manner by varying the time slot assignment between frames. Due to the interrupted nature of each link, it is easy to change its parameters (time slot location, time slot length, and modulation rate) between frames.
In FDMA with FA, non-uniform static assignment of frequency slots can be easily implemented, by simply dividing the bandwidth available into non-uniform portions and assigning the portions as required. However, dynamic reassignment of bandwidth (DA) can be implemented easily only in a non-efficient manner and only when a contiguous slot of wider bandwidth becomes available. It should be noted that when several non-adjacent narrow slots become available, these are not usable to make up a broad band channel. It is traditionally considered impossible to change the frequency of a link while it is in operation, without interrupting the transmission and reacquiring the transmission at the new frequency.
This problem is addressed in “Theory and Performance of Frequency Assignment Schemes for Carrier with Different Bandwidths under Demand Assignment SCPC/FDMA Operation,” by Chiba, Takahat and Nohara, TEICE Trans. Commun. Vol. E75-B, No. 6, Jun. 6, 1992, which is incorporated herein by reference. This reference proposes to reduce the statistical probability of the problem by an assignment algorithm. At best, this is a partial solution to the problem.
In “Airline—the Flexible Radio Access System,” by Siddiqui and Challoner, Ericson Review No. 3, 1996 and in “Next Generation Broadband Flexible Radio Access Network Solution,” IBC Conference on Wireless in the Local Loop, London 1996, a system for dynamic allocation in a system with non-uniform bandwidths is proposed. The bandwidth is changed by a “make-before-break” transfer from one frequency band to another. First, a parallel link is set up. When the parallel link is stable, the transmission is transferred to the parallel link and the original link is broken.
The make-before-break mechanism has a number of disadvantages. First, it is not frequency efficient, at least in the short term, when a channel occupies two bands at the same time. Second, the remote station transmitter must be capable of transmitting on two channels at the same time, i.e., they must have twice the power capability that they really need, a very expensive requirement. Third, since the remote station transmitter must transmit two signals at the same time, it must be extremely linear to avoid inter-modulation effects. Such systems are more expensive and consume more power. Finally, the remote station must contain two modems rather than one.
Therefore, there is a need in the art for improvements in bandwidth allocation in communication systems.